This application is a national stage of PCT International Application No. PCT/EP2008/009402, filed Nov. 7, 2008, which claims priority under 35 U.S.C. §119 to German Patent Application No. 10 2007 054 826.7, filed Nov. 16, 2007, the entire disclosure of which is herein expressly incorporated by reference.
The invention relates to a fuel cell system, in which a compressor is connected via a line to a humidifier, which in turn is connected via a second line to a fuel cell stack, at which a waste gas line is provided. The humidifier supplies moisture to the compressed air, conventionally ensuring better operation of the fuel cell stack. However, the moisture is disadvantageous at cold ambient temperatures, since it may lead to icing. It is therefore desirable sensible to remove moisture from the fuel cell stack at the end of operation of a fuel cell system. As far as possible the fuel cell stack should first be preheated on restarting, before moisture is supplied.
It is known from German patent document DE 60 2004 000 440 T2 and Japanese patent document JP 2005-259458 A to cause the compressor to run in the reverse direction after stopping operation of the fuel cell system, so that the compressor sucks air out of the fuel cell stack, coming in particular from the waste gas line, instead of supplying precompressed air to the fuel cell stack, so generating a vacuum in the fuel cell stack and causing water to evaporate.
However, in practice it has proven difficult to cause a compressor simply to run in the reverse direction, bringing about a pumping action. A conventionally constructed compressor suffers from damage if it is simply operated in reverse relative to the conventional operating direction.
One object of the invention, therefore, is to provide a fuel cell system of the above-stated type in which it is possible to work with conventional compressors while nonetheless achieving a good cold starting capacity.
This and other objects and advantages are achieved by the fuel cell system according to the invention, in which there is a connection between a compressor and a waste gas line, and a valve arrangement serves to open up precisely this connection. The valve arrangement is designed simultaneously to close off the first line. By opening up the connection, air compressed by the compressor is passed via the waste gas line to the fuel cell stack. By blocking the first line, the air is in any case caused to bypass the humidifier.
The invention is based on the recognition that the same effect as that of a pump may be achieved when the air continues to be compressed to an elevated pressure but is supplied in reverse manner. A concrete option for achieving the invention is to provide a specific valve arrangement.
If the humidifier is a gas-to-gas humidifier, which is usually the case and is preferred when applying the invention, the waste gas line is also connected to the humidifier, continues in the humidifier itself and then leads outside the humidifier into an outlet for the waste gas. It is then possible to provide a valve arrangement which comprises a first three-way valve in the first line (i.e., between compressor and humidifier) and a second three-way valve in the waste gas line. Between the first three-way valve and the humidifier a connection to the outlet may lead off, in which a further valve is then arranged. The air then actually flows through the entire arrangement in the reverse direction, is passed from the compressor to the waste gas line, from the waste gas line through the fuel cell stack, from the fuel cell stack to the second line, via which air is otherwise supplied, then to the humidifier. Because the air leaves the humidifier via the first line, it has to pass by way of the connection to the outlet, where it may exit.
In the prior art described initially, the compressor is operated in the reverse direction so as to function as a pump after stopping operation of the fuel cell system, for the purpose of evaporating water. The present invention now makes it possible to supply dry air even while restarting or upon stopping operation despite the presence of a humidifier, specifically in a start phase or in a stop phase, by passing air to the fuel cell stack via the air discharge line.
It should be noted that, in the method according to the invention, a conventional compressor is preferably used, which needs not necessarily be operable in the reverse direction. This works in that a compressor compresses air both in the start and stop phases and during conventional operation, by the same given mode of operation. Start and stop phases and conventional operation are distinguished only by the manner in which the valves of a valve arrangement are switched.
A further advantage of the invention is that, even during the drying phase, (i.e., when dry air is supplied to the fuel cell), the fuel cell may still generate power in a considerable quantity up to the maximum possible value.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.